BEV, PHEV or HEV: The Differences Affect the Architecture

Momentum behind electric vehicle development is at an all-time high, with strong consumer interest prompting OEMs large and small to transition their vehicles to electric propulsion. But with the buzz comes a lot of confusion.

What is the difference among the various vehicle types? How do these vehicles get recharged, and how far can they run before needing a charge? How does the vehicle type influence architectural decisions? Here are the essentials.

Vehicle types

The global market for electric vehicles (EVs) is a small portion of the total automotive market, but that is expected to change, and fast. According to Boston Consulting Group, the global market share of EVs grew from 8 percent in 2019 to 12 percent in 2020 – and the firm expects that EVs will account for more than half of all light vehicles sold by 2026.

Electric vehicle is actually an umbrella term. In fact, there are several types of EVs. All use electricity for at least some of their operation, but that is pretty much where the resemblances end. Today there are four main types, listed here by reliance on battery power:

• Mild hybrid electric vehicle: MHEVs improve gas mileage by using a modest 48V battery and electric motor to increase the efficiency of their internal combustion engine (ICE). Unlike a hybrid electric vehicle, an MHEV does not run solely on electric power, although the ICE can be shut off during braking, coasting and stopping. In addition, even when the ICE is switched off, the electric motor can power nonessential features such as air conditioning or seat heaters.
  

• Hybrid electric vehicle: HEVs are the most common type of hybrid, and they have been around the longest, too. HEVs have two power drives: a fuel-based engine and an electric motor with a larger battery. When the car starts, it first rolls under electric power. Then, as soon as the vehicle achieves speed, the gas engine kicks in. An onboard computer system determines when electricity or gas should be used. Also, users do not plug in an HEV. Through a process known as “regenerative braking,” the car’s electric battery gets a little recharge every time the driver touches the brakes. The Toyota Prius is the most well-known HEV.

• Plug-in hybrid electric vehicle: PHEVs split the difference between battery electric vehicles (BEVs, described below) and HEVs. Like BEVs, PHEVs have an electric motor that is recharged via an external plug. And like HEVs, they also have a fuel-based ICE. One big difference from HEVs is that a PHEV can travel a decent distance on electric power alone — about 20 to 30 miles — due to their increased battery size and ability to recharge from the grid. Examples of PHEVs include variants of the Toyota Corolla and RAV4, the Volvo XC40 and the BMW 3-Series.
  
  
• Battery electric vehicle: BEVs are powered entirely by electricity, meaning a BEV has no ICE, no fuel tank and no exhaust pipe. Instead, it has one or more electric motors powered by a larger onboard battery. Users charge the battery via an external outlet. BEVs currently exist in several forms, including cars, buses, motorbikes and scooters, and even boats.
  

A fifth category exists, but it is still mostly a work in progress: fuel-cell electric vehicles. An FCEV runs on a battery-powered electric motor, but with a difference — it is charged with hydrogen, instead of electricity, and produces its own electricity.

How the architecture changes

Several architectural differences are involved in moving from one of these types of EV to the next.

The most obvious is battery capacity, which is measured in kilowatt hours (kWh). Since the electric motor in a MHEV is only assisting a gas engine, it might have a battery that is 1 kWh or less. An HEV’s battery has to deliver enough power to run the vehicle for brief periods, so it might be as large as 8 kWh, while a PHEV could have a battery as large as 15 kWh to drive farther on electric power only.

The battery in a full BEV has to power everything in the vehicle, all the time, so typical BEV capacities range from about 40 kWh to 80 kWh, although some are now emerging with batteries as large as 200 kWh.

The biggest architectural change comes when moving from HEV to PHEV. That is the point where outside power — from the grid, possibly from an outlet at the vehicle owner’s house — now enters the vehicle. Receiving alternating current (AC) power from the outlet means the vehicle has to have an inlet, leading to an onboard charger that converts the power to direct current (DC) to charge the battery. As the battery powers devices in the vehicle, the power stays DC but requires an inverter to change to AC to power the electric motors.

The PHEV always has a gas engine to fall back on and has a smaller battery, so conventional charging will likely be sufficient to charge it in a short amount of time. PHEVs can typically charge in a couple of hours with a Mode 2 charging cord – that is, one that can plug into a standard household outlet.  

For BEVs, this method could take more than a day to fully charge their higher-capacity batteries, so many BEV owners opt to install a wall-mounted EV charger in their home and/or use public charging stations, including fast-charging stations that can potentially charge the BEV in under one hour.

Consequently, when moving from a PHEV to a BEV, this change in charging speed becomes a consideration that drives architectural decisions. While BEVs remove all ICE components, which reduces complexity, the requirement for faster charging means adding DC charging capabilities.

High-speed charging means higher power. While ICE vehicles use 12-volt batteries, BEV batteries often deliver 350 volts, with some OEMs even going as high as 800 volts. Higher power requires larger and more robust cables and connectors to allow for faster charging.

The voltage has to be stepped down for different components — especially electronics that run on 12V or less — but overall this is an efficient approach that saves energy. OEMs are discovering that even mild hybrids can achieve efficiencies by using 48V for higher-power components such as air conditioners or active-suspension systems and then stepping down voltage for electronics. Using 48V also makes it seamless to turn the engine on and off when starting and stopping.

All of these pieces — the batteries, converters, inverters, wires and connectors — add weight and complexity, but OEMs can reduce that complexity with a thoughtful approach that takes into account the full electrical/electronic architecture, such as Aptiv’s Smart Vehicle Architecture™.

Selecting and purchasing an electric vehicle involves trade-offs and decisions that, assuming all goes well, will lead to a big payoff in ensuring greener mobility. That is true for designing and building one, too.